New drag and drop API (#4571)

This commit implements a new API for drag and drop, with a `DataTransfer` type which abstracts over the various clipboard/drag and drop APIs across different platforms. I built this on top of #2429 although admittedly I ended up removing pretty much all of their work while I was reworking the design.

This is being built in order to help support [drag-and-drop work](https://github.com/slint-ui/slint/issues/1967) in Slint's winit backend. As part of that work, I did extensive research on how drag-and-drop and clipboard APIs are implemented across different platforms, and wrote a (still WIP) research document that can be found [here](https://gist.github.com/eira-fransham/06750cf8d25ade08d362a0ca8dfafe06).

The new API is inspired by the browser's [`DataTransfer`](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer) API.

winit-win32/src/window_state.rs

@@ -22,7 +22,7 @@ use winit_core::keyboard::ModifiersState;
 use winit_core::monitor::Fullscreen;
 use winit_core::window::{ImeCapabilities, Theme, WindowAttributes};
 
-use crate::{SelectedCursor, event_loop, util};
+use crate::{SelectedCursor, WindowAttributesWindows, event_loop, util};
 
 /// Contains information about states and the window that the callback is going to use.
 #[derive(Debug)]
@@ -154,6 +154,7 @@ pub enum ImeState {
 impl WindowState {
     pub(crate) fn new(
         attributes: &WindowAttributes,
+        _win_attributes: &WindowAttributesWindows,
         scale_factor: f64,
         current_theme: Theme,
         preferred_theme: Option<Theme>,